Re: Disallow USING clause when altering type of generated column - Mailing list pgsql-hackers

On Wed, Aug 21, 2024 at 4:57 PM Peter Eisentraut <peter@eisentraut.org> wrote:
>

+ /*
+ * Cannot specify USING when altering type of a generated column, because
+ * that would violate the generation expression.
+ */
+ if (attTup->attgenerated && def->cooked_default)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_TABLE_DEFINITION),
+ errmsg("cannot specify USING when altering type of generated column"),
+ errdetail("Column \"%s\" is a generated column.", colName)));
+

errcode should be ERRCODE_FEATURE_NOT_SUPPORTED?

also
CREATE TABLE gtest27 (
    a int,
    b text collate "C",
    x text GENERATED ALWAYS AS ( b || '_2') STORED
);

ALTER TABLE gtest27 ALTER COLUMN x TYPE int;
ERROR:  column "x" cannot be cast automatically to type integer
HINT:  You might need to specify "USING x::integer".

should we do something for the errhint, since this specific errhint is wrong?